Scott blogged Martian SlingShot
, a Bonjour-enabled network syncing utility for your Macs, back in January, and a recent 2.0 update has brought iTunes music and playlist syncing to the table. In addition to being able to 'publish' and 'subscribe' to folders between multiple Macs on a local network (also known as a 'live backup'), SlingShot can now keep iTunes libraries and playlists in sync, watching for changes and copying over files when new additions are found.
A 30-day demo is available that allows you to publish or subscribe to one folder or playlist. Fortunately, Martian Technology LLC allows you to purchase one license for $29.99 USD on all the Macs you own, while business licensing is available upon request.